refactor proxy handling code (#2357)

- allow gallery-dl proxy settings to overwrite environment proxies
- allow specifying different proxies for data extraction and download
  - add 'downloader.proxy' option
  - '-o extractor.proxy=–PROXY_URL -o downloader.proxy=null'
    now has the same effect as youtube-dl's '--geo-verification-proxy'

gallery_dl/downloader/http.py

@@ -121,7 +121,8 @@ class HttpDownloader(DownloaderBase):
             try:
                 response = self.session.request(
                     "GET", url, stream=True, headers=headers,
-                    timeout=self.timeout, verify=self.verify)
+                    timeout=self.timeout, verify=self.verify,
+                    proxies=self.proxies)
             except (ConnectionError, Timeout) as exc:
                 msg = str(exc)
                 continue